Filters are used in a variety of electronic circuits to filter out certain frequency components of a signal while letting other frequency components pass. For example, in communication circuits filters may be used to block frequency components outside a frequency band or part of a frequency band used for communication and to be processed by further circuits
To increase bandwidth, communication standards like wireless communication standards (e.g. LTE, Long-Term Evolution) or also wire-based communication standards continually increased a used frequency range and a number of used frequency bands. In communication devices implementing such standards, often highly selective filters matched to the respective frequency bands are required. The frequency bands used may differ from country to country. Therefore, a plurality of filters having different filter characteristics (for example different passbands) is required. Furthermore, in what is referred to as carrier aggregation several frequency bands are operated at the same time. This requires specific filter designs for exactly those combinations. With a specific filter provided for each possible combination, the number of physical filters is actually much higher than the number of available bands. In order to reduce the number of different filters (two-port up to n-port filters) actually required in a communication device, tunable filters are highly desirable.
As highly selective band pass filters in communication circuits and devices, surface acoustic wave (SAW) or bulk acoustic wave (BAW) technologies are frequently used. Conventional filters of such types are designed for fixed resonance or center frequencies. As a consequence, many filters are required to serve individual frequency bands or aggregated combinations of several frequency bands used in current communication standards like LTE, including WiFi. Radio frequency (RF) switches are then used to select individual filters of the plurality of filters for example for a desired signal path between an antenna and a low noise amplifier or power amplifier. Therefore, in such conventional approaches a large number of mostly discrete components is required, increasing circuit complexity, radio frequency losses, manufacturing complexity and the needed space for radio frequency front ends in communication devices. Space is limited in mobile devices such as smartphones, and therefore tunable solutions are highly desired to save area.
Some approaches have been made to make such SAW or BAW filters tunable in order to decrease the overall number of filters required. However, conventional tuning techniques may have drawbacks regarding their tuning range, their selectivity and/or regarding losses introduced by the possibility of tuning the filter. Therefore, it is an object to provide improved possibilities for filter tuning.